Conduits under pressure of large cross section, for example forced conduits, are often constructed using sections of metallic tubes, of circular cross-section, placed one behind the other and sealingly connected in a manner which makes it possible to withstand the prevailing pressure. The tube may consist of simple arched ferrules assembled by welding or, if high pressures are involved, it may consist of a thick cast wall. The tubular elements are then provided at their ends with members for joining by fitting together or with flanges applied onto one another by clamping means with interposition of leaktight seals.
In order to effect the welds under good conditions, the wall must not be too thick, and above all the tubes must be properly aligned. The means for joining by fitting together permit the tolerance of certain misalignments, but only within relatively narrow limits. Moreover, it is difficult to construct thick-walled metallic tubes having very large sections and, in practice, the diameter of such conduits does not exceed 56 inches, which corresponds to about 1.5 m.sup.2.
In order to obtain larger cross sections, it is possible to use tubes made of reinforced or prestressed concrete, but such elements are extremely heavy and bulky and, for very large sections, their transportation and installation become very difficult. In addition, the concrete tubes are only able to withstand relatively low pressures.
Moreover, when a conduit of circular section is buried in the ground, it rests essentially along the lower generatrix, which is therefore highly loaded. It is also possible to support a metallic conduit on appropriately shaped cradles: however, the latter must be spaced apart from one another, and the conduit is subjected to bending stresses.
The applicant has previously proposed other embodiments of buried conduits using prefabricated elements whose dimensions remain within the limits permitting road transportation and which may be assembled on site. Such arrangements, which form the subject, for example, of European Patents 081,402 and 0,188,487, permit the construction of hollow tubular structures of very large sections which may, if desired, be used to convey fluid provided there is an appropriate leaktight lining. However, such embodiments require handling elements of very large dimensions and of great weight, and it is relatively difficult to achieve, on laying, perfect alignment of these elements in order to ensure sufficient leaktightness at elevated pressures.